Leaking Toilet Mansfield

We have a old Mansfield 210 series toilet. A few days ago, our toilet started leaking and flushing automatically. So I looked online and found that many peoples had same issue, but they resolved it by simply changing the rubber gasket. I tried it, but water was still leaking into the toilet with the new gasket. So I decided to change the entire mechanism, which includes a tube with new gasket. Despite that the water is leaking from the tank into toilet. I don't understand where the leak is, whole assembly is new. But, with old Mechanism with new gasket, leakage was quite a bit. But, with new mechanism with new gasket leakage stopped almost 90%, but it's still leaking. Has anybody dealt with similar problem before?

Never had an issue with new gaskets leaking. The gasket does sit in a groove, make sure it's in there properly and the tank is clean where the gasket seals.